There are known in the art wave-motion-type mechanisms wherein a mechanism similar to that of a cup-type wave gear mechanism is used as a linear-actuation mechanism for converting rotational movement into linear reciprocating movement. A linear actuator of this type is disclosed in Patent Document 1. The linear actuator disclosed in Patent Document 1 comprises a circular spline having a screw groove in an inner peripheral surface thereof, a flexspline having a screw thread on an outer peripheral surface thereof, and a wave generator. When the wave generator rotates, the locations in which the circular spline and the flexspline are threaded move in a circumferential direction, and rotational movement is converted into linear movement.
Specifically, a cup-shaped flexible screw having a lead screw formed on an outer peripheral surface thereof is used in place of a cup-shaped flexspline in a cup-type wave gear mechanism; and a circular nut threaded onto the lead screw is used in place of the circular spline, the circular nut having a screw groove that is formed in an inner peripheral surface thereof. The flexible screw is bent into an elliptical shape by the wave generator, and threads into the circular nut at either end location of a major axis of the ellipse. When the wave generator rotates, the threading locations move in the circumferential and axial directions. As a result, the rotational movement of the flexible screw is converted to move the circular nut in a linear reciprocating manner.
[Patent Document 1] JP-A 2007-154955
In a conventional wave-gear-type linear-actuation mechanism, a flexible, thin-walled screw is used; therefore, the holding force in the axial direction is limited by the flexible screw. Specifically, the force for holding the circular nut in a prescribed position in the axial direction against an external force is limited by the buckling strength of a diaphragm part of the cup-shaped flexible screw, and the shear strength of the lead screw part.